What type of "lick tub" are we talking about here? What are we looking to provide?

There are protein lick tubs and there are mineral lick tubs. My Mineral-lyx tubs are only around 4% protein but have a good range of vitamins and trace minerals. 

My dealer also sell protein tubs that are 20-30%  protein.

Corn may be a good source of energy but if your soils are short of selenium, copper, etc. corn grown on those soils will not provide those trace minerals.

There are a wide range of lick tubs depending on the goal.